1. Field of the Invention
The present invention relates to apparatus and methods enabling simultaneous reading out of transponders, and in particular such methods and apparatus enabling simultaneous reading out, or reading, of passive inductive transponders arranged in a stack, so that the coils of the transponders are closely magnetically coupled to each other.
2. Description of Prior Art
With reference to FIG. 3, a conventional transponder system will be described below which consists of a base station 10 and a transponder 12, depicted schematically in FIG. 3. In the simplified representation, the base station 10 includes a power source 14, a resistor 16 serially connected to the power source, and a capacitor 18 connected in parallel to the serious connection consisting of the power source 14 and the resistor 16. Connected in parallel to the capacitor 18 is a transmitter coil 20. Along with the capacitor 18, the coil 20 thus represents a parallel resonant circuit loaded, in the simplified example, by the resistor 16 and the power source 14, and in reality by the power output stage.
The transponder 12 includes a transponder coil 22 and a tuning capacitor 24. The transponder coil 22 and the tuning capacitor 24 define a parallel resonant circuit. In addition, in the simplified schematic representation of the transponder 12, a rectifier diode 26 is connected between the parallel resonant circuit and a capacitor 28 for storing energy. A variable load, consisting of a resistor 30 and a switch 32, is arranged in parallel with the capacitor 28. Further, a transponder ASIC 34 (ASIC=application-specific integrated circuit) is provided which is connected to the switch 32 in the manner represented.
The base station 10 inductively supplies the transponder 12 with energy 36, whereas the transponder returns transponder data 38 by means of the load modulation method. Here, the transponder is a passive transponder not requiring any additional source of energy.
In the base station 10, a current in the transmitter coil 20 generates an alternating magnetic field 40 at an operating frequency fB. The transponder coil 22 of the transponder 12 is arranged in the near-field region of the transmitter coil 20, so that the alternating magnetic field 40 impinges upon the transponder coil 22 and induces an electrical voltage there. The voltage induced serves the transponder ASIC 34 as an operating voltage obtained via the rectifier 26 and the capacitor 28.
To send data, the transponder ASIC 34, controlled by means of a data signal 42, switches the resistor 30 to be parallel with the capacitor 28 via the switch 32, i.e. the transponder ASIC 34 switches on the variable load. This leads to an increase in the power consumption of the transponder. Due to this power consumption of the transponder, the field generated by the transmitter coil 20 is weakened. As a consequence, the voltage applied at the resistor which exists between the transmitter coil and the power output stage and is shown schematically as a resistor 16 in FIG. 3, increases in the base station 10. These voltage changes in the base station 10 may be detected to reconstruct the data signal 42 sent by the transponder 12, such a method being referred to as load modulation.
In a transponder system as has been explained above with reference to FIG. 3, the voltage induced in the transponder coil 22 must be higher than the operating voltage of the transponder ASIC. To enable this, the tuning capacitor 24 is connected in parallel with the transponder coil 22, the tuning capacitor forming a resonant circuit with the transponder coil 22. In the transponder system shown, the resonant frequency of the transponder resonant circuit is the operating frequency fB, so that the voltage induced has a local maximum at the operating frequency fB depending on the frequency.
If several transponders exist in the field of the transmitter coil of a base station, and if these transponders are closely magnetically coupled to each other, the transponders mutually influence each other. In addition to affecting the quality of the respective transponder resonant circuits, this influence also relates to the resonant frequency of the transponder resonant circuits, so that same no longer matches the operating frequency. So far it has not been possible to power and read out a stack of passive inductive transponders which are arranged, in relation to each other, such that the coils of same are closely magnetically coupled.
The present invention is based on the object of providing apparatus and methods enabling transponders arranged in a stack to be powered and read out.
In accordance with a first aspect, the present invention provides a transponder reading device for reading out a plurality of inductive passive transponders having a resonant circuit, the device comprising:
a transmitter coil for generating an alternating magnetic field having an operating frequency fB, in which the plurality of transponders may be placed such that they are magnetically coupled to each other; and
a retuning means arranged in the alternating magnetic field and having such a frequency-dependent impedance that a voltage induced in the resonant circuits of the transponders by the alternating magnetic field has a maximum in the range of the operating frequency fB.
In accordance with a further aspect, the present invention provides a method for reading out a plurality of inductive passive transponders having a resonant circuit, the method comprising:
generating an alternating magnetic field at/with/having an operating frequency fB;
placing the plurality of transponders in the alternating magnetic field such that the transponders are magnetically coupled to each other;
placing a retuning means in the alternating magnetic field, the retuning means having such a frequency-dependent impedance that a voltage induced in the resonant circuits of the transponders by the alternating magnetic field has a maximum in the range of the operating frequency fB; and
receiving data created by the transponders in response to the voltage induced.
The present invention is first of all based on the findings that, if several transponders are arranged in the field of the transmitter coil and if same are closely magnetically coupled to each other, the circuit elements at the terminals of each transponder coil are mapped at the terminals of every other transponder coil. This leads to an electrical network containing coupled coils. This network is no longer a simple resonant circuit. The voltage induced no longer takes on a maximum at the operating frequency and is therefore no longer high enough to power the transponder ASIC.
Instead, in a plurality of transponders arranged in close magnetic coupling to each other in the alternating magnetic field of the transmitter coil, a maximum of the voltage or the current, respectively, induced in the transponder coils is caused by each transponder coil, i.e. by each transponder oscillating circuit. These maximums occur at various frequencies, the frequency positions of all maximums changing as soon as a further transponder is added in the alternating magnetic field. If the transmitter coil is also part of an oscillating circuit, as in the example described above with reference to FIG. 3, a maximum of the voltage induced in the transponder coils is caused also by the oscillating circuit of the transmitter coil.
As has been mentioned above, the various oscillating circuits influence each other, so that, depending on the number and the nature of the transponder coils stacked, the voltage maximums have frequency positions deviating from the resonant frequencies to which the individual transponder resonant circuits are tuned. The transponders thus cannot be read out. Since the transponders have fixed device variables, for example a transponder coil inductance of 5 xcexcH and a tuning capacitance of 2 pF, it is not possible to access the resonant frequency of the resonant circuits of the transponders. In addition, there are statutory specifications for the operating frequency fB, so that same cannot be changed either, a common operating frequency for transponder systems being 13.56 MHz.
In accordance with the invention, a retuning means is therefore introduced into the near field of the transmitter coil such that the amplitudes of the voltages induced in the transponder coils, and the currents induced which are proportional to these voltages, take on a local maximum at the operating frequency fB, i.e. the frequency of the transmitting voltage, and are sufficiently high to enable powering of the transponder ASIC. In accordance with the invention, the retuning means introduced into the proximity of the transponder stack and thus into the near field of the transmitter coil preferably comprises one or several coils magnetically coupling to the transponder coils. For creating a resonant circuit, a capacitor may be connected to the coil of the retuning means. Additional network elements, for example resistors or further coils, may also be connected to the coil.
Due to the retuning means introduced into the alternating magnetic field of the base station, the voltages and/or currents induced in the transponders exhibit an additional maximum, the frequency position of which may be set by a corresponding selection of the frequency-dependent impedance of the retuning means. Thus, the frequency position of this additional maximum may be set to coincide with the operating frequency fB, so that all transponders may be read out at the operating frequency. By the introduction of the retuning means into the alternating magnetic field, the other maximums of the voltage or the current, respectively, induced in the transponder coil will be shifted, it being possible to shift one of these maximums to the operating frequency fB. This additional maximum is used for a small number of transponders in the transponder stack, whereas a shifted maximum is used for a large number of transponders in the transponder stack.
In accordance with preferred embodiments of the present invention, the frequency-dependent impedance of the retuning means is preferably variable, provisions preferably being made for means for varying the frequency-dependent impedance until all transponders in a transponder stack respond. It is thus possible to ensure a safe readout of all transponders in a transponder stack without knowing the number of transponders in the stack.
In accordance with a further aspect, the present invention further provides a transponder means with a resonant circuit consisting of a transponder coil and a tuning capacitor, the resonant circuit being tuned such that a lower minimum cut-off frequency is in the range of a preset operating frequency fB of an alternating magnetic field, the lower minimum cut-off frequency fu,min being the frequency approached by the lowest-frequency maximum of the voltage induced in the transponder resonant circuit by the alternating magnetic field of the operating frequency fB if a theoretically unlimited number of transponder means are arranged in the alternating magnetic field.
In accordance with the above-mentioned third aspect, the present invention is based on the findings that the frequency, at which the absolute maximum of the voltage or the current, respectively, induced occurs, drops and approaches a lower limit, i.e. a lower minimum cut-off frequency, as the number of transponders in the transponder stack increases. If the transponder resonant circuits in such a transponder stack are dimensioned and/or tuned such this lower minimum cut-off frequency occurs at the operating frequency fB, it is possible to read out the plurality of transponder means without requiring any retuning means. In a different approach, one of the plurality of transponder means may be considered the retuning means in such a case.
Further developments of the present invention are set forth in the dependent claims.